Effect of speciation on the apparent diffusion coefficient in nonreactive porous systems

A combined theoretical and experimental study of the effect that concentration and ionic speciation have on the apparent diffusion coefficient is performed using a nonreactive porous material in a divided cell diffusion apparatus. Varying the ionic species concentration over two orders of magnitude changes the apparent diffusion coefficient by no more than 20% for the systems studied. By contrast, at fixed ionic concentration, varying the ionic species changes the initial apparent diffusion coefficient by a factor of two. Over longer periods of time, the apparent diffusion coefficient varies in time, increasing by a factor of ten or more. For one system, the macroscopic diffusion potential across the specimen induces a transient negative apparent diffusion coefficient; iodide ions are transported from regions of low iodide concentration to regions of high iodide concentration. The theoretical analysis shows that, in nonreactive porous systems, the behavior of all the concentrations and species studied can be completely characterized by an electro-diffusion system of equations that contain two time-independent constants: the porosity and the formation factor. The relationship between these results and the prediction of concrete performance in the field is discussed.